1. Field of the Invention
This invention relates generally to the field of hard disc drives, and more particularly, but not by way of limitation, to a flexure for supporting a head in a disc drive, the flexure having a reduced unloaded height.
2. Brief Description of the Prior Art
Disc drives of the type known as "Winchester" disc drives are well known in the industry. Such disc drive data storage devices typically contain a stack of rigid discs coated with a magnetic medium on which digital information is stored in a plurality of circular concentric tracks. The storage and retrieval of data--also called "writing" and "reading", respectively--is accomplished by an array of heads, usually one per disc surface, which are mounted on an actuator mechanism for movement from track to track. The most common form of actuator used in the current generation of disc drive products is the rotary voice coil actuator, which uses a voice coil motor (VCM) coupled via a pivot mechanism to the heads to access data on the disc surfaces. The structure which supports the heads for this movement is referred to as a head/gimbal assembly, or HGA.
The HGA in a typical disc drive consists of three major components:
1. a slider, which features a self-acting hydrodynamic air bearing and an electromagnetic transducer for recording and retrieving information on a spinning magnetic disc. Electrical signals are sent to and received from the transducer via very small twisted copper wires; PA1 2. a gimbal, which is attached to the slider and is compliant in the slider's pitch and roll axes for the slider to follow the topology of the disc, and is rigid in the yaw and in-plane axes for maintaining precise slider positioning, and; PA1 3. a load beam, which is attached to the gimbal and to a mounting arm which attaches the entire assembly to the actuator. The load beam is compliant in the vertical axis to, again, allow the slider to follow the topology of the disc, and is rigid in the in-plane axes for precise slider positioning. The load beam also supplies a downward force that counteracts the hydrodynamic lifting force developed by the slider's air bearing.
Since the introduction of the first Winchester disc drive, the physical size of the slider has been progressively reduced, first from the original Winchester head to the so-called "mini-Winchester", and more recently to the 70 and 50 Series heads, which are 70% and 50% the size, respectively, of the mini-Winchester slider. While these size reductions are significant, the overall vertical dimension of the HGA has been dictated more by the slider-supporting mechanism than by the size of the slider itself.
The load beam and gimbal comprise an assembly generally known as a head suspension, head flexure, or simply a flexure. An example of such a flexure is described in U.S. Pat. No. 4,167,765.
Historically, the gimbal and load beam are fabricated discretely. The gimbal and load beam pieces are realized by chemically etching 300 series stainless steel foil into the desired shape, and then the two pieces are attached by means of laser welding.
The general technology trend in disc drive data storage devices is continual shrinking of the physical size of the product while providing increased data storage capacity. The down-sizing of the product has required smaller components, especially the principal components such as discs, sliders and flexures. Additionally, disc drive designers seek to add capacity to their designs by incorporating as many discs as possible within defined package dimensions. As the number of discs in the unit increases, the spacing between the discs decreases, thus further driving the need for smaller sliders and flexures.
Another industry trend is to provide the user of disc drives with high data storage capacity at low cost. This requires developing improved data recording technology and finding lower cost ways of manufacturing the components of the disc drive.
The use of discrete gimbal and load beam components laser welded together, as shown in the '765 patent, has become problematic in disc drives of the current 2.5", 1.8" and 1.3" generations of disc drives. In such units, the flexures must become thinner in order to allow desirable close spacing of the discs, while the overlapping required to laser weld two discrete components dictates a certain minimum height for the flexure.
Furthermore, the use of thinner gimbal and load beam components increases the likelihood of residual stress caused by the laser welding of the two components together. It has been found that laser welding produces residual tensile stress in the material local to the welds. This causes the flexure to distort. In the longitudinal direction, the flexure curls from the residual weld stress, and this makes it more difficult to fit the flexure between closely spaced discs during the manufacturing process. Further, if the welds are not placed symmetrically about the longitudinal centerline of the flexure, the residual weld stress will cause a torsional distortion, or twisting, of the flexure. Such an flexure is undesirable since the twist will create a moment, or torque, on the slider's air bearing, causing unwanted changes in the flying attitude of the head, and potentially rendering the assembly unusable.
The welding process is also a substantial portion of the labor that goes into the manufacture of a flexure, and it would, thus, be advantageous to eliminate the practice of making discrete gimbals and load beams and welding the two together for cost reduction.
Since the gimbal and load beam components must overlap in flexures of existing art, the emphasis on reducing the thickness of the flexure assembly has most often focused on reducing the thickness of the individual gimbal and load beam components. The thickest area of the load beam is the region known as the rigid beam, which usually features flanges along the outer edge along the longitudinal axis of the flexure. U.S. Pat. No. 4,996,616 teaches how a pair of drawn ribs can provide reinforcement of the rigid beam section of the flexure. Unfortunately, the drawn pair of ribs of '616 requires that the flexure material be strained to exceedingly high levels. Such stain can introduce cracks in the drawn material, and high stresses in the material near the ribs.
Various attempts have been made to solve the problems inherent in welding a gimbal and load beam together by devising a flexure in which the gimbal and load beam are formed from a single piece of material and would thus require no welding. An example of such an integrated gimbal and load beam is presented in U.S. Pat. No. 4,245,267. A second example is known as the HTI Type 16, or T16, manufactured by Hutchinson Technology, Incorporated. Both of these flexures have a gimbal incorporated into the load beam and, of course, no gimbal-to-load beam welds. Both include a bonding surface on which adhesive is placed to secure attachment of the slider to the flexure. A plurality of beams, etched into the load beam, connects this bonding surface to the load beam portion of the flexure and provides the desired gimbal characteristics.
One failing of the flexure of the '267 patent and the T16 flexure relates to an element of flexure design commonly referred to as "load point". Simply stated, load point refers to the single point of contact where the downward force of the load beam is applied to the slider. Proper selection of this load point ensures that the forces related to the hydrodynamic air bearing of the slider are properly balanced. In prior art flexures such as the one described in the '765 patent, load point is developed by forming an upward-extending dimple in the gimbal bonding surface. The load beam contacts the spherical surface of this dimple at a single point to allow proper gimbal action. In the case of the '267 and T16 flexures, however, a well defined load point is not provided, and, thus, an undesirably wide range of variation in slider flying characteristics is associated with these types of flexure.
A second fundamental problem with the '267 and T16 types of flexures is that the downward force of the load beam is applied to the slider by placing the gimbal beams into bending mode, and the gimbal beams must therefore be stiff in bending mode. These same gimbal beams, however, must be compliant in bending mode to allow the proper gimballing action. This conflicting requirement results in designs that either work poorly as a gimbal or become deformed under load.
A third problem with the '267 and T16 flexures is that the slider bonding surface, in general, covers a large area over the center of the slider. The slider is attached to the flexure with an adhesive epoxy, and, in order to reduce the cure time of the adhesive, the assembly is usually heated in an oven. Since the slider and flexure are made of dissimilar materials with different coefficients of linear thermal expansion, thermally induced strains develop at the bond when the assembly cools. These strains can distort the slider and undesirably change the flatness of the air bearing surface of the slider, thus, once again, introducing unacceptably wide variation into the flying characteristics of the heads.
Two examples of a unitary, or one-piece, flexure which overcomes these deficiencies are described in co-pending U.S. patent applications No. 07/975,352, of which this application is a continuation-in-part, and No. 07/976,163, now U.S. Pat. No. 5,331,489, issued Jul. 19, 1994, both filed Nov. 12, 1992, both assigned to the assignee of the present invention and the latter of which is incorporated herein by reference.
The flexures of the above-cited references are manufactured from a single piece of fully hardened 300 series stainless steel using the processes of through-etching and half-etching. That is, the overall outline and through openings are created by through-etching, while certain features are formed with a reduced material thickness brought about by the process of half-etching.
In typical chemical through-etching processes, the material to be etched is first coated on both sides with a material called resist. The resist is patterned using a stencil and exposing the resist to a light source. Unexposed resist is then stripped away, leaving exposed metal that will be etched away in the presence of an acid-like etchant, while those areas of the material protected by the resist, or "mask", remain at their original thickness. Both sides of the material are treated in this manner, with the pattern on both sides being identical and very accurately aligned. By carefully controlling the strength of the etchant and the time of exposure of the material to the etchant, very precisely shaped and dimensioned parts can be realized.
In half-etching, the pattern of the stencil on one side of the material is dissimilar to that on the other side. This also is a well known technique for etching text, art or half-tone photographs into sheet metal. It is known that if the area to be half-etched is large--that is, it has a length or diameter many times that of the material thickness--then the depth of the half-etching will be approximately sixty percent that of the material thickness. That is, during the time of immersion in the etchant solution which will cause through-etching in those areas where the etchant-resistant mask is missing on both sides of the material, those areas of the material which are exposed only on one side will be etched to about forty percent of the original material thickness.
This half-etching process is used in the flexures disclosed in the cited references to reduce the thickness of a pair of gimbal beams which are compliant in the flexure's roll and pitch axes, and stiff in the yaw and in-plane axes.
Two tabs are also formed in the disclosed flexures, with the first tab left at the original material thickness and used to adhesively bond the slider to the flexure.
The second tab is used to support and mount the load point button which contacts the top of the slider and transfers the downward force of the flexure to the slider to counterbalance the hydrodynamic lifting force of the slider's air bearing. This load point tab is half-etched on the side toward the slider, except in that location selected for the load point, which retains the original material thickness. This is achieved by masking the location and shape of the desired load point button to prevent etching at that point, as described above.
Because of the relative thinness of the load point tab of the cited references, this tab must be pre-formed--that is, bent at a compensating angle--so that when the entire assembly is placed under the designed load in cooperative arrangement with the surface of a disc, the load point tab is brought back into parallel relationship to the gimbal beams and slider mounting tab. Such a head/flexure assembly has a very low "loaded" height.
It has been found, however, that the design of the flexures of the cited references introduces significant variability between individual units when produced in a high-volume manufacturing environment. Specifically, it is difficult to closely control the thickness of the load point tab as determined by the half-etching process, which leads to the necessity of varying the angle at which the load point tab is preformed to compensate for the variations in thickness.
Additionally, since the load point tab is pre-bent to compensate for load on the head, the unloaded height is necessarily increased, and it is this unloaded height of the head/flexure assembly which determines how much inter-disc spacing must be allowed for assembly of the disc drive.
A need clearly exists, therefore, for an improved slider-supporting flexure which reduces the overall unloaded vertical height of the HGA, as well as eliminates much of the individual variation between units, and which can be manufactured in a simple, cost-effective manner.